Changed plans from 300 to 600. Needed a new modem to get the higher speed so I bought the Motorola MB7621 (certified by Comcast). I had the new modem provisioned for 600 but I'm not able to see anything above 280. It's like I'm stuck on the old 300 even though the plan clearly shows Extreme Pro 600. I've called & received a credit because they can't send a tech out. Anybody have suggestions or ideas on what the problem may be? Thanks.
Is there a router / WiFi in the mix here ? If so, for a test, what speeds do you get with a computer hardwired directly to the MB7621 cable modem ?
Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.
Good luck !